Saturday, November 26, 2011

Snow or no? Discussion on the possibility of Mid-South November snow

It seems much too early in the year to be discussing a real threat of snow in the forecast (as opposed to wish-casting a storm in the model output 2 weeks out).  But it appears Mother Nature wants us Mid-South meteorologists to fine-tune our winter forecast skills early this year as she is throwing a doozy at us! Not so much in the way of a big snowfall, but in the complexity of the factors that could come together to bring us our first flakes of the winter of 2011-2012!

We'll start with the "easier" part. A potent cold front stretching from southern Canadian into the Gulf of Mexico will move through the Mid-South overnight tonight, tapping an abundance of moisture that will produce widespread rainfall, mainly from evening through the overnight hours. In addition, a weak surface low will form in the Lower Mississippi Valley, which could provide enough lift for some scattered rumbles of thunder and periods of heavy rainfall. Rainfall amounts of up to 2"+ are not out of the question tonight.

Surface map valid at 6am Sunday showing a cold front near the Tennessee River and  low pressure over northeast MS

24-hour projected rainfall totals from HPC/NWS through 6am Sunday
As low pressure develops along the front, it will effectively stall out just to our east and low pressure will also form in the upper air pattern behind the front.  The complexity in the forecast deals with the strength, position, and movement of the upper low.  The forecast graphics below are from the generally-preferred model this morning, the European model (or ECMWF), showing the expected position of the low Monday morning and Tuesday morning.  The upper low is right over the Mid-South Monday morning and shifts east to the TN Valley Tuesday morning.

The European model shows upper level low pressure centered over the Mid-South on Monday morning
The European model shows the upper level low moving east into the TN Valley on Tuesday morning
The position of the low is not the only factor in a snow forecast, however.  Far from it!  The amount of available moisture, strength of the low, and degree of cold air throughout the atmosphere (from surface on up) all play a major role.  And in November, things have to be pretty much perfect to get accumulating snow south of the Mason-Dixon line!

Computer models don't necessarily know that though and can spit out some pretty incredible forecasts, such as the overnight run of the NAM from last night (shown below), which brings Memphis a couple of inches of snow on Sunday night!  The very next run of the same model (this morning) indicates less than 1" for the Memphis area and 8-10"+ for middle TN!  (I wouldn't bet on either one, by the way... fickle models!)  This is what we meteorologists get to deal with and why southern snow forecasts can be the hardest of them all!

Total snowfall forecast from the NAM midnight model run showing 4"+ over Memphis... not likely
For comparison, the Saturday morning NAM model run, which now shows <1" for Memphis and 8"+ for middle TN
We'll be watching the models closely over the next 48-72 hours, especially watching for more agreement among them, which would lead to a higher confidence forecast.  For the time being, the MWN Forecast carries a chance of a rain/snow mix late Sunday night/early Monday with another chance Monday night as the low pulls away.  Due to warm ground temperatures (it's over 70 degrees today), I would be surprised if more than a little sticks on grassy surfaces by Monday morning.  Given that there is a chance of snow, you can BET on cold temperatures!  We'll see the mercury fall through the 40s Sunday and perhaps remain in the 30s all day on Monday!

Further updates to the blog can be expected through the weekend.

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